1. Field of the Invention
This invention is characterized by a gas filled hollow core chalcogenide photonic bandgap fiber and its use.
2. Description of Related Prior Art
In the Raman method, a photon interacts with a molecule and excites the molecule to a higher energy vibrational state. In the process, the incident photon loses energy and is converted to a lower frequency photon. Normally, this process converts a small fraction of the incident photons of the pump to lower frequency photons. The generated lower frequency photons are the Stokes wave. In stimulated Raman scattering, however, the Stokes wave is of high enough intensity to stimulate the scattering of other photons, resulting in the conversion of a large fraction of the pump beam to the Stokes beam. The frequency shift of the pump beam is dependent upon the vibrational or rotational modes of the molecules comprising the media.
Raman conversion in highly nonlinear gasses, such as hydrogen, is a well known technique for generation of light in the mid-infrared. In this technique, a high peak power pump beam is focused down into a Raman active gas, usually contained in a large gas cell. The pump beam is Stokes shifted by the gas to generate the mid-infrared light. Very efficient conversion of near-infrared to mid-infrared light has been seen, however, the short interaction lengths in the bulky gas cells require high peak power sources for efficient conversion.
Raman conversion in gas filled hollow dielectric waveguides has also been demonstrated. Here, the dielectric waveguides increase the interaction lengths and thus higher efficiency and lower thresholds are possible than in the free-space gas cell methods. Note, however, that very high peak powers, in the kilowatts, are still necessary to reach threshold due to the large mode area, poor mode quality, and high losses of the dielectric waveguides.
Recently, Raman conversion in gas filled hollow core silica photonic bandgap glass has been demonstrated in the visible spectrum. Here, a silica photonic bandgap structure was filled with hydrogen gas and a pump beam at 532 nm was focused into the gas filled silica photonic bandgap fiber and both the Stokes wave at 682 nm and anti-Stokes wave at 435.2 nm were observed. Due to the small mode area, good mode quality and relatively low losses, thresholds of about 933 watts peak power were seen.
At least two issues limit the applicability of silica-based photonic bandgap fibers in the infrared. First, the silica glass matrix of the photonic bandgap fiber is highly absorbing in the infrared beyond about 2 microns. In an ideal photonic bandgap structure with an infinite number of hole layers, the propagating modes can be contained entirely in the air defect region. Practically, however, the finite number of hole layers, variations in hole periodicity and deformation of the air hole size and shape results in penetration of the mode field into the cladding. This can result in significant loss for signals at wavelengths that are highly absorbed by the photonic bandgap glass matrix, and thus efficient Stokes conversion beyond the transmission window of silica is not possible. Note, too, that propagation of high power pump or Stokes wave at these wavelengths can lead to heating and damage of the fiber.
Second, for commercially available silica photonic bandgap fiber, there exists a single large bandgap which is used for propagation. While the bandwidth of this gap is large, it would not be large enough to allow a single Stokes shift of a near infrared pump from 1-2 microns to the mid-infrared. This limits the applicability of silica-based gas filled hollow core bandgap fiber for infrared Raman lasers and the like.
Until recently, it was not known whether infrared transmissive materials, such as chalcogenide glass, could be fabricated into photonic bandgap fiber and whether the structures would demonstrate bandgaps that could be used for guidance in the infrared. In fact, recently it was shown that guidance in the infrared is possible in As—S and As—Se based photonic bandgap structures. The high transparency of the As—S and As—Se glasses in the infrared allows very low loss air core propagation of infrared light. Furthermore, in some structures there exist several widely space bandgaps which can be used for pump and Stokes wave propagation. Through careful design of these with appropriate transmission bands and choosing suitable Raman active gases, hollow core photonic bandgap Raman lasers in the infrared are feasible.